1. Field of the Invention
This invention relates to a gain control circuit for a pulse width modulation amplifier and, more particularly, to an improved gain control circuit which controls a gain by varying a supply voltage to a pulse amplification circuit of the pulse width modulation amplifier in accordance with a control signal without varying a level of an input signal to be amplified.
2. Description of the Prior Art
In an amplifier employing a pulse width modulation circuit and adapted for use in such as an audio signal amplifier, it is known in the art to control a gain of the amplifier in such a way as a level of an input signal applied to the amplifier is adjusted to a certain level so as to obtain a desired output level at the output of the amplifier. That is, the adjustment of the input signal level is accomplished by controlling a variable resistor or the like which is provided to an input side of the amplifier and devides the input signal level.
A typical arrangement, in the art, of an amplifier with a pulse width modulation circuit (hereinafter called as PWM amplifier) as shown in FIG. 1 comprises: a pulse width modulation circuit 1 for modulating a carrier signal by an input signal as a modulating signal, i.e., an audio input signal to be amplified in the PWM amplifier in the case of audio signal amplification thereby producing a pulse width modulated signal; a pulse amplification circuit 2 for amplifying the pulse width modulated signal; and a lowpass filter circuit 3 through which the amplified pulse width modulated signal delivered from the pulse amplification circuit 2 is passed to generate an output signal having the same waveform as the original modulating signal. More specifically, the pulse width modulation circuit 1 which is comprised of a comparator circuit receives at a non-inverting input terminal 4 a carrier signal having generally a sawtooth waveform as shown in FIG. 1, and at an inverting input terminal 5 the input signal. The pulse width modulation circuit 1 then produces the pulse width modulated signal by comparing the input signal to the carrier signal. The pulse width modulated signal thus outputted from the pulse width modulation circuit 1 is applied to the pulse amplification circuit 2 which comprises a driver stage circuit 6 followed by a switching circuit 7. The switching circuit 7 is formed with a pnp transistor 8 and an npn transistor 9 each having a diode coupled between the collector and the emitter of the transistors 8 and 9, respectively. The collectors of these transistors 8 and 9 are connected together to a junction point 10 to which the input terminal of the low-pass filter circuit 3 is connected. The output terminal of the filter circuit 3 is connected to an output terminal 16 of the PWM amplifier to which a load 11 is connected.
Thus, the input signal applied to the inverting input terminal 5 of the pulse width modulation circuit 1 is compared to the carrier signal to generate the pulse width modulated signal which is fed to the driver stage circuit 6 in order to drive the switching circuit 7. The output of the driver stage circuit 6 is pulse-amplified by the transistors 8 and 9 of the switching circuit 7 so that the amplified pulse modulated signal is provided to the input of the low-pass filter 3. In this case it should be noted that the switching circuit 7 is set to be operable in ON-OFF mode in association with the output level of the driver stage circuit 6 with the result that the amplified pulse modulated signal has such an amplitude as is substantially equal to, but is slightly lower than power supply voltages supplied at power supply terminals 14 and 15, respectively, because of a collector-emitter saturation voltage. Finally, in the low-pass filter 3, the low frequency components of the amplified pulse modulated signal is extracted therefrom to develope across the load 11 the output signal, i.e., an amplified audio signal having the same waveform as the input signal.
In the PWM amplifier thus constructed, a conventional method for controlling the gain of the PWM amplifier is accomplished by providing adjusting means such as a variable resistor 12 provided in such a manner that an input signal to the PWM amplifier is applied to the inverting input terminal 5 of the pulse width modulation circuit 1 through the variable resistor 12. The variable resistor 12 has a slider terminal and two stationary terminals, the former being connected to the inverting terminal 5 of the pulse width modulation circuit 1, and the latter being connected to an input supply terminal 13 and ground, respectively. Hence, the input signal supplied to the termnal 13 is divided by the variable resistor 12 and delivered to the inverting input terminal 5 of the circuit 1 so as to be compared to the carrier signal. The output of the circuit 1 is pulse-width-modulated in accordance with the level of the input signal at the inverting input terminal 5, the output signal of the filter 3 is varied as desired by changing a position of the slider contact of the variable resistor 12.
The use of such a variable resistor, however, gives rise to some problems because it is often necessary to locate the variable resistor remotely from an input terminal of the associated PWM amplifier in order to manipulate it outside a casing of an audio amplifier. This results in the need for an appreciable lengthy lead wires which couple the variable resistor to the input terminal of the PWM amplifier, so that the wires are liable to receive noise interference and the location of the variable resistor is limited to a restricted place so as not to receive noise interference. Moreover, the power supply voltages applied to the switching circuit is substantially maintained constant during its operation regardless of the level of both the input signal to be amplified and the carrier signal. Therefore, there is another problem that even in the case of low signal amplification spurious radiation from the switching circuit is kept almost equal to that in the case of high signal amplification, and that carrier leakage is further developed by raising the power supply voltages in order to produce a high output voltage across the load.